1. Field of the Invention
The present invention relates to an image reading apparatus which is capable of reading images by means of a plurality of image sensors.
2. Related Background Art
Image sensors ordinarily used in reading images can be sorted into two types: namely, silicon crystal type sensors such as CCDs and bipolar-type sensors, and thin-film type sensors such as CdS film sensors and amorphous silicon sensors. On the other hand, optical systems used in image reading apparatus can also be sorted into two types: namely, image contraction type and real-size type. As to color image reading apparatus, there are two types of color separation methods. One of these methods is to use a single image sensor while selectively using one of a plurality of light sources or one of a plurality of color filters. In another method, an image is read simultaneously in different colors, followed by a color separation.
There are several types of method which rely upon simultaneous reading of image in different colors. In one of these methods, a plurality of image sensors sensitive to different colors are arranged in a side-by-side fashion. In another method, a single-line image sensor is striped with filter elements of different colors so that color-separated signals are provided in a time-sharing manner.
In designing an image reading apparatus, various types of methods described hereinabove are selected in accordance with the performance of the image reading apparatus to be obtained. For instance, when a specifically high reading speed is demanded, an image sensor of the thin-film type, which inherently has a high reading speed, is preferably used. Similarly, when a specifically high sensitivity is used, an image sensor of real-size type, which provides a comparatively large light-receiving area for a given reading resolution, is used preferably.
A color image reading apparatus in particular requires an image sensor having a high sensitivity, considering that the quantity of light received is decreased because of the presence of a color separation filter and that the image sensor itself has a certain spectral sensitivity characteristic. Therefore, for the purpose of conducting high-speed reading under illumination by a practically available light source device, the image sensor most preferably used is a sensor of real-size silicon crystal type sensor with stripe filter. The assignee of the present invention already has proposed image reading apparatus which make use of a real-size type image sensor mentioned above, in U.S. Pat. Nos. 4,734,787, 4,691,114 and in U.S. patent application Ser. No. 193,227, allowed Jun. 30, 1989, a continuation of application Ser. No. 804,110.
As a matter of fact, however, it is extremely difficult to produce a single-chip silicon crystal image sensor capable of covering the full length (297 mm) of A4-size paper, mainly for the reason of restrictions in the manufacture. Under this circumstance, a single-line sensor capable of performing high speed reading has been proposed in which a plurality of sensor elements of real-size silicon crystal type are connected in series and physically arranged suitably so as to constitute a line sensor.
Such a single-line image sensor constituted by a plurality of image sensor elements connected in series in the direction of main scan can suitably be used in reading color images. It is assumed here that a color separation stripe filter is composed of blue (B), green (G) and red (R) filters each having a reading resolution on the order of 16 dots/mm. In such a case, the pitch of pixels is represented by 1/16 mm.times.1/3=20.8 .mu.m. This means that the series of sensor elements have to be positioned and assembled at a precision on the order of this small pitch of pixels. According to current technique, however, there is no substantial difficulty in attaining this order of precision in assembling the sensor elements. It is even possible to obtain a single-chip line sensor having a higher reading precision. In this type of single-line image sensor, however, a problem is encountered in regard to the difference in the reading density between different sensor elements, attributable to fluctuation in the characteristics. In particular, deviation in colors is a serious problem in a color image reading apparatus employing this type of sensor. The deviations in image density and colors between different sensor elements are mainly attributable to (1) difference in sensitivity and dark current level between different sensor elements, and (2) fluctuation in the characteristics of signal processing circuits for different sensor elements or for filters of different colors.
The assignee of the present invention also has proposed, in a U.S. patent application filed on Nov. 11, 1987, a method which provides matching of outputs from different sensor elements while eliminating any influence of fluctuation of characteristics between the image sensor elements. This method, however, is still unsatisfactory in that the matching between levels of outputs from the image sensor elements cannot be attained to a satisfactory level.
An apparatus has been proposed in U.S. Pat. No. 4,653,111, assigned to the same assignee, as this invention for displaying data of image read by an image pickup apparatus. This apparatus has a drawback in that it does not have any means for enabling the user to know the state of shading correction performed by a shading correction means.